This invention relates generally to bandpass filters suitable for use at very high and ultra high frequencies and, more particularly, to a filter composed of cascaded helical resonators with non-uniform spacing between resonant circuit elements.
It is well known that a space completely enclosed by conducting walls can contain oscillating electromagnetic fields within it and will possess certain resonant characteristics. Resonators of this type are commonly termed cavity resonators and are extensively used for filtering applications. A helical resonator is one member of a family of slow-wave transmission lines often used in the very high and ultra high frequency ranges and consists of a helix placed within a metal cavity. This form of resonator provides a minimum practical resonator size for a given frequency and hence is very useful in applications where compactness is essential.
FIG. 1 illustrates such a prior art helical resonator bandpass filter. Here a series of helical resonators, one being shown at 10, are positioned within cavities, will all cavities being the same size and being aligned in cascade. A series of apertures, one of which is shown at 12, provide for a predetermined coupling between the resonators. One end of each coil, such as coil 10 at 16, is directly connected to the metal cavity whereas the other end is floating (open) in fixed facial relationship to an adjustable screw 14 to allow controlled capacitive loading of the coil. Thus the distributed inductance of each coil, with its adjustable and distributed capacitance to the cavity, form a resonant circuit, tunable within a narrow range, within the cavity. When the resonator cavity cross-section is square, the distance between the resonators is a constant.
In order to increase the bandwidth of this type of resonant filter, the coupling between the resonators can be increased. A widely used technique for accomplishing this increase in coupling is to widen the aperture between resonators. A problem with such prior art resonators is that, even if the apertures between elements are increased to the maximum (i.e. the partitions are completely removed), the bandwidth obtained is still too narrow for many applications.
Another approach utilizes adjustable capacitive coupling between resonators. Still another type of filter uses a resonant filter with coupling between the elements adjusted by rotating those elements relative to each other. Both of these two approaches uses an adjustable coupling between elements rather than the fixed coupling preferred for many filter applications.
It has also been suggested that for coaxial resonators, increased coupling, and thus increased bandwidth, can be obtained by foreshortening resonators in one dimension thus placing the resonant stages closer together. This, however, results in a degradation of the resonator unloaded Q's and an increase in insertion losses.